Root Hypoxia Reduces Leaf Growth

Abstract

This study examined the potential role of restricted phloem export, or import of substances from the roots in the leaf growth response to root hypoxia. In addition, the effects of root hypoxia on abscisic acid (ABA) and zeatin riboside (ZR) levels were measured and their effects on in vitro growth determined. Imposition of root hypoxia in the dark when transpirational water flux was minimal delayed the reduction in leaf growth until the following light period. Restriction of phloem transport by stem girdling did not eliminate the hypoxia-induced reduction in leaf growth. In vitro growth of leaf discs was inhibited in the presence of xylem sap collected from hypoxic roots, and also by millimolar ABA. Disc growth was promoted by sap from aerated roots and by 0.1 micromolar ZR. The flux of both ABA and ZR was reduced in xylem sap from hypoxic roots. Leaf ABA transiently increased twofold after 24 hours of hypoxia exposure but there were no changes in leaf cytokinin levels.Exposing roots of a fast-growing Populus trichocarpa x P. deltoides hybrid (Hybrid 11-11) to oxygen-depleted conditions in solution culture reduced the growth rate of the leaves (24). The reduction in growth rate was generally detectable within 1 h and did not recover while the roots were hypoxic. The duration of leaf growth was similar to plants with aerated roots, with final leaf size reduced by 35 to 60%.The rate of expansion of leaf cells is determined by cell turgor or driving force relative to wall extensibility and yield threshold (14). In the leaves of hypoxic Populus plants, the reduced growth rate was not due to reduced turgor (24). Growth was probably limited by alterations in wall characteristics, since wall extensibility decreased in the leaves of plants with hypoxic roots (24). Cell wall extensibility and yield threshold are dynamic characteristics ofthe wall and are under metabolic control (6). A stimulus from hypoxic roots could alter these properties in leaves of Populus plants. In this way, the status of the roots could be relayed to the leaves to reduce growth.Previous reports of shoot responses to root hypoxia have suggested several routes by which a stimulus could be sent to the leaves and have hypothesized the nature of the stimulus (3,4,12,30). The plant growth substances ABA and cytokinins have been suggested as possible regulators of leaf growth.

Aquaporin-Mediated Reduction in Maize Root Hydraulic Conductivity Impacts Cell Turgor and Leaf Elongation Even without Changing Transpiration

“…In anoxia-susceptible plant species, including maize, anoxia induces modifications in root metabolism or photosynthesis (Geigenberger, 2003). Smit et al (1990) proposed along these lines that in poplar, transport of a root chemical stimulus to the shoot may be necessary to cause a reduction in leaf elongation. Yet, neither abscisic acid nor cytokinin concentrations were changed in growing leaf tissues.…”

“…On this time scale, changes in turgor appear to determine most of the leaf elongation responses. In longer term studies, the hydraulic component may be overridden by osmotic adjustments (Van Volkenburgh and Boyer, 1985), growth-depressive effects of hormones such as cytokinins (Smit et al, 1990) or ethylene (Shiu et al, 1998), or changes in cell wall rheology.…”

Stomatal Closure in Flooded Tomato Plants Involves Abscisic Acid and a Chemically Unidentified Anti-Transpirant in Xylem Sap

“…But subsequent work (Jackson et al, 1988) failed to confirm that oxygen-deficient roots of peas were a rich source of ABA (Castonguay et al, 1993) for the shoots. Moreover, flood-induced stomatal closure and increased foliar ABA were not prevented by grafting shoots of pea or tomato onto root systems of ABA-deficient mutants (Jackson, 1991), and ABA delivered from roots to shoots in xylem sap decreases rather than increases after 24 h of flooding (Smit et al, 1990;Else et al, 1995b). A third hypothesis was suggested by Zhang and Zhang (1994), who attributed increased levels of ABA in younger leaves of flooded pea plants to synthesis and export from older leaves in association with a loss of turgor and wilting caused by waterlogging the soil.…”

Adaptações morfofisiológicas da soja em solo inundado

“…A deficiência de O 2 prejudica a síntese de fitorreguladores, tais como giberelinas e citocininas (Smit et al, 1990).…”